Purpose: To assess the distribution of dark-light changes in iris features and associated factors amongst American Caucasians, American Chinese and mainland Chinese aged 40 years and over.
Methods: Four gender- and age-matched cohorts: Caucasians, American Chinese, Southern Chinese and Northern Chinese comprised the study population. Anterior segment optical coherence tomography (ASOCT) images were acquired under dark and light conditions. Customized software was used to calculate iris thickness at 750 μm from the scleral spur (IT750), iris curvature (ICurv), iris area (IArea), and pupil diameter (PD) from ASOCT images.
Results: Data from 121, 124, 121 and 120 subjects were available for American Caucasian, American Chinese, Southern and Northern Chinese subjects respectively. Caucasians had significantly less dark-light change (Δ) of IT750 and greater ΔPD than Chinese. When using multiple linear regression analyses, the inter-ethnic difference for ΔIT750 was independent of age, gender, refractive error, anterior chamber width and axial length. After controlling for ΔPD, ΔICurv was found to be inversely associated with age (β = -0.001, P = 0.001). Independent predictors of greater ΔPD included younger age (β = -0.02, P = 0.001), Caucasian ethnicity (β = -0.022, P = 0.009) and wider anterior chamber (β = 0.18, P = 0.03).
Conclusions: Compared with Caucasians, Chinese irides show greater thickening with light to dark adaptation despite the lesser change of PD in Chinese. The change of iris area per unit change of pupil diameter from dark to light did not differ between ethnicities. These dynamic characteristics intrinsic to Chinese irides may partially account for the substantially increased risk for angle closure among this ethnic group.